nfsv4: bitmap should implement Iterable interface

to make it useful in for-each loop.
Added bitmap#set and bitmap#isSet.
GETATTR/SETATTR use bitmap as an Iterable

Acked-by: Gerd Behrmann
nfsv4: consult export file on getRootInode()

NFSv4 do not have mount operation. As a result any host
can establish TCP connection and NFSv41 session. Nevertheless,
we can reject access to root of file system if there are no exports
for the client.

Acked-by: Gerd Behrmann